Windows Phone 8.1

Hi Rand, I made some changes in plugin.xml and add BluetoothLePlugin.cs. App with this plugin can communicate with BLE device which has characteristic with notification. I am going to make some changes for improve code. Now one can already start to used it. I would be grateful for your comments.
P.S. It is my first pull request so I think I made something wrong. :) I just copied plugins for android and iOS from your repos to my project, made wp8.1 plugin and commit.  Later I'll post the project files.